Thin film transistors (TFT) are broadly used as switching elements for display devices such as liquid crystal display, etc. A cross sectional structure of a typical conventional TFT is shown in FIG. 10. As shown in FIG. 10, TFT comprises a gate electrode and an insulator layer in this order on the substrate, and further, comprises a source electrode and a drain electrode formed above the insulator layer having a predetermined distance between them. Over the insulator layer exposing between the electrodes, a conventional semiconductor layer is formed having partial surfaces of each electrodes. In TFT with such a structure, the semiconductor layer forms a channel region and an electric current flowing between the source electrode and the drain electrode is controlled by a voltage applied to the gate electrode resultantly causing an On-Off operation.
Conventionally, TFTs were fabricated employing amorphous silicon or polycrystalline silicon, however, there were problems that making screens large in display devices or so with the use of TFTs is accompanied by significantly soaring in manufacturing cost because a chemical vapor deposition (CVD) equipment used for the preparation of TFTs employing the silicon is very expensive. Further, because a film-forming process of the amorphous silicon or the polycrystalline silicon is carried out under an extremely high temperature, causing a limitation in kinds of the material employable as a substrate for TFT, there was the problem that a lightweight polymer film substrate or so is unemployable.
For the purpose of overcoming such a problem, a TFT with the use of an organic substance replacing the amorphous silicon or the polycrystalline silicon is proposed. With regard to the film-forming process for fabricating a TFT employing organic substances, a vacuum vapor deposition process or a wet-coating process is well known. Those film-forming processes enable not only to realize making screens large in display devices while suppressing soaring in manufacturing cost but also to relatively reduce a process temperature required for film-forming. Accordingly, a practical use of the TFT employing an organic substance is highly expected because of an advantage in little limitation in a selection of material for a substrate and as a result, a large number of report about TFT employing an organic substance are published. Examples of the report include Non-Patent Literatures 1 to 19 below.
Further, with regard to the organic substance employable in an organic compound layer of TFT, a multimer such as conjugate polymer or thiophene (refer to Patent Literatures 1 to 5 below, etc.); metallophthalocyanine compound (refer to Patent Literature 6 below, etc.); or condensed aromatic hydrocarbon such as pentacene (refer to Patent Literatures 7 and 8 below, etc.) is used singly or as a mixture in combination with another compound each other. With regard to the materials of n-type FET, for example, Patent Literature 9 below discloses 1,4,5,8-naphthalenetetracarboxyldi unhydride (NTCDA), 11,11,12,12-tetracyanonaphth-2,6-quinodimethan (TCNNQD), 1,4,5,8-naphthalenetetracarboxyldiimide (NTCDI), etc.; and Patent Literature 10 below discloses phthalocyanine fluoride. Additionally, although Non-patent Literature 16 teaches that oligophenylenevinylene exhibits transistor characteristic, it does not disclose at all what kind of structure oligophenylenevinylene has. Because a home page of Lucent Technologies in United States of America (http://www.lucent.com/press/0902/020925.bla.html) concludes that a person among the main authors of the Non-patent Literature 16 forged data and because Non-patent Literature 17 withdraws the content of Non-patent Literature 16, the present invention never gets any influence from Non-patent literature 16. Further, although Non-patent Literature 18 mentions about TFT property of soluble phenylenevinylene, its electron mobility is extremely as small as about 10−5 cm2/Vs because it has a long-chain alkyl group in its center position. Furthermore, although Non-patent Literature 19 mentions about electron mobility of phenylenevinylenepolymer (poly-paraphenylenevinylene (PPV)), it is also so small as 10−4 cm2/Vs that any practical performance is not achieved. Namely, because PPV being a high molecular compound has a long main chain structure, a turbulence in crystal structure induced from bending or molecular weight distribution of the main chain structure reduces electronic field-effect mobility to the small values. On the other hand, although Patent Literature 11 reports about a preparation of a TFT device by obliquely vapor depositing a liquid crystal compound, a TFT device having superior performance without depending upon the liquid crystal compound or upon a film-forming process such as the obliquely vapor deposition process was eagerly desired.
On the other hand, there is an organic electroluminescence (EL) device as a device similarly using an electric conduction. However, the organic EL device generally forces to feed charges by applying a strong electric field of 106 V/cm or greater across a thickness direction of a ultra-thin film of 100 nm or thinner, whereas it is necessary for the organic TFT to feed charges for several μm or longer with high-speed under an electric field of 105 V/cm or smaller and accordingly, an enhanced electric conductivity becomes necessary for the organic substance itself. Despite the above circumstances, the conventional compounds in the organic TFT had problems in fast response as transistor because its capability for moving electrons was poor, because a field-effect mobility of electron was small, and because response speed was slow. Further, On/Off ratio was also small. The above On/Off ratio is defined as a value obtained by dividing an amount of an electric current flowing between a source and a drain when some gate voltage is applied (On) by an amount of an electric current flowing there when any gate voltage is not applied (Off). A word “On electric current” usually means an amount of a (saturated) electric current at a time when the electric current between the source and the drain saturates while increasing the drain voltage.                Patent Literature 1: Japanese Unexamined Patent Application Laid-Open No. Hei 8-228034        Patent Literature 2: Japanese Unexamined Patent Application Laid-Open No. Hei 8-228035        Patent Literature 3: Japanese Unexamined Patent Application Laid-Open No. Hei 9-232589        Patent Literature 4: Japanese Unexamined Patent Application Laid-Open No. Hei 10-125924        Patent Literature 5: Japanese Unexamined Patent Application Laid-Open No. Hei 10-190001        Patent Literature 6: Japanese Unexamined Patent Application Laid-Open No. 2000-174277        Patent Literature 7: Japanese Unexamined Patent Application Laid-Open No. Hei 5-55568        Patent Literature 8: Japanese Unexamined Patent Application Laid-Open No. 2001-94107        Patent Literature 9: Japanese Unexamined Patent Application Laid-Open No. Hei 10-135481        Patent Literature 10: Japanese Unexamined Patent Application Laid-Open No. Hei 11-251601        Patent Literature 11: Japanese Unexamined Patent Application Laid-Open No. 2005-142233        Non-patent Literature 1: F. Ebisawa et al. Journal of Applied Physics, vol. 54, p 3255; 1983        Non-patent Literature 2: A. Assadi et al. Applied Physics Letter, vol. 53, p 195; 1988        Non-patent Literature 3: G. Guillaud et al. Chemical Physics Letter, vol. 167, p 503; 1990        Non-patent Literature 4: X. Peng et al. Applied Physics Letter, vol. 57, p 2013; 1990        Non-patent Literature 5: G. Horowitz et al. Synthetic Metals, vol. 41-43, p 1127; 1991        Non-patent Literature 6: S. Miyauchi et al. Synthetic Metals, vol. 41-43; 1991        Non-patent Literature 7: H. Fuchigami et al. Applied Physics Letter, vol. 63, p 1372; 1993        Non-patent Literature 8: H. Koezuka et al. Applied Physics Letter, vol. 62, p 1794; 1993        Non-patent Literature 9: F. Garnier et al. Science, vol. 265, p 1684; 1994        Non-patent Literature 10: A. R. Brown et al. Synthetic Metals, vol. 68, p 65; 1994        Non-patent Literature 11: A. Dodabalapur et al. Science, vol. 2568, p 270; 1995        Non-patent Literature 12: T. Sumimoto et al. Synthetic Metals, vol. 86, p 2259; 1997        Non-patent Literature 13: K. Kudo et al. Thin Solid Films, vol. 331, p 51; 1998        Non-patent Literature 14: K. Kudo et al. Synthetic Metals, vol. 102, p 900; 1999        Non-patent Literature 15: K. Kudo et al. Synthetic Metals, vol. 111-112, p 11; 2000        Non-patent Literature 16: Advanced Materials Vol. 13, No. 16, p 1273; 2001        Non-patent Literature 17: Advanced Materials Vol. 15, No. 6, p 478; 2003        Non-patent Literature 18: W. Geens et al. Synthetic Metals, Vol. 122, p 191; 2001        Non-patent Literature 19: Lay-Lay Chua et al. Nature, Vol. 434, March 10 issue, p 194; 2005        